Event Center is not designed to use as an audio only conferencing service but WebEx Personal Conference (PCN). However, a host can schedule an event and enable the following options:
a) Attendees can join X minutes before the scheduled start time
b) Attendees can also connect to audio conference
This will allow the users to start/join the audio conference and communicate with each other.
IMPORTANT: The option "Mute upon entry for all participants" should not be enabled while scheduling the event.
Hope this helps!!!